Electric utility companies and power consuming industries have in the past employed a variety of approaches to metering electrical energy. Typically, a metering system monitors power lines through isolation and scaling components to derive polyphase input representations of voltage and current. These basic inputs are then selectively treated to determine the particular type of electrical energy being metered. Because electrical uses can vary significantly, electric utility companies have requirements for meters configured to analyze several different nominal primary voltages, the most common of which are 96, 120, 208, 240, 277 and 480 volts, root mean squared (RMS).
Electric utility meters employing electronic components instead of electromechanical components have become more widely used in the industry. The use of electronic components including microprocessor components have made electricity metering faster and more accurate. Of course, the meters typically receive and monitor alternating current (AC) power from the power distribution system, though usually, direct current (DC) power is required to operate the meter's electronic components. Therefore, electronic meters use power supply devices to generate DC power from the already-available and constantly-present AC line voltage. As discussed in U.S. Pat. No. 5,457,621, which is incorporated herein by reference, power supply devices have been created to generate the required microprocessor DC power regardless of the value of the available line voltages (e.g., 96 to 480 volts RMS).
Typically an electric meter power supply device works over the extended range of input AC voltage and develops sufficient output power to supply a variety of communication options within the meter. The power supply device also must withstand severe input voltage transients and meet requirements regarding acceptable levels of conducted electromagnetic interference, such as those set out by the Federal Communications Commission (FCC) and the International Special Committee on Radio Interference (CISPR) of the International Electrotechnical Commission (IEC).
A power supply is able to process an extended range of input voltages and typically includes, among other things, devices that store electrical charge (e.g., capacitors), a switching device in electrical connection with the device that stores electrical charge, and a transformer in electrical connection with the switching device (hereinafter “switching transformer”). FIG. 1 depicts an example embodiment of an electrical circuit of a power supply 100 in a typical commercial or industrial electric meter. Power supply 100 is capable of operating over a wide range of input voltage, which may range from approximately 46 to 530 volts AC (VAC). After the input voltage is rectified by a rectifier 110, two or more devices that store electrical charge 115, 120 directly filter the wide range of rectified direct current voltage (VDC), which may range from approximately 65 to 750 VDC. A switching device 130 and a switching transformer 140 each may handle a wide range of the filtered VDC. This large voltage range creates significant design challenges for power supply components such as devices that store electrical charge 115, 120, switching device 130, and switching transformer 140.
Devices that store electrical charge 115, 120 handle the operating high voltages plus transient voltages. Because of the potential high voltages, the devices must be physically large. For example, such devices 115, 120 each may be capacitors rated for 10 microfarad (μf). Additionally, devices 115, 120 may be associated with resistors 116, 121 to help ensure equal division of voltages across devices 115, 120.
Because of the relatively large size of devices 115, 120, at lower operating voltages, more capacitance is used than is required when the input voltage is at the upper extremes of the voltage range. Therefore power supply 100 has more energy available at high input voltages but loads cannot make use of it. Additionally, because of the size of devices 115, 120, and the nature of switching transformer 140, the layout of the printed circuit board is usually dominated by the combination of devices 115, 120 and switching transformer 140.
To meet the wide voltage range, a cascade circuit such as shown in FIG. 1 may be required to divide the high direct current voltage between a switching device 125 (e.g., a transistor) and a switching regulator. Switching device 125, which may be a metal oxide semiconductor field effect transistor (MOSFET), typically will operate at the switching frequency of switching regulator 130 and handle the full switching current of switching transformer 140. Switching device 125 shares approximately half of the input voltage during periods when the input voltage is above a certain threshold, such as, for example, above 400 volts.
Designing switching transformer 140 to operate over this wide voltage range is also difficult. Usually, to accommodate the wide voltage range, switching transformer 140 operates in continuous mode to meet the low voltage conditions and must provide large primary inductance values to limit the rate of rise of current during high voltage conditions.
Therefore, in a power supply of an electric utility meter, there is a need for a reduction of the wide range of available line voltages that are applied to the components of the power supply so that power supply design may be simplified and the size of the components may be reduced.